Subscriber identification using Option 82 field

Option 82, or the relay information option is specified in RFC 3046, DHCP Relay Agent Information Option, and allows the router to append some information to the DHCP request that identifies where the original DHCP request came from.

There are two sub-options under Option 82:

• Agent Circuit ID Sub-option (RFC 3046, section 3.1); this sub-option specifies data which must be unique to the box that is relaying the circuit

• Remote ID Sub-option (RFC 3046 section 3.2); this sub-option identifies the host at the other end of the circuit. This value must be globally unique

Both sub-options are supported and can be used separately or together.

Inserting Option 82 information is supported independently of DHCP relay. However, in a VPLS service (when DHCP Relay is not configured), DHCP snooping must be enabled on the SAP to be able to insert Option 82 information.

When the circuit id sub-option field is inserted, it can take following values:

sap-id

the SAP index (only under an IES or VPRN service)

ifindex

the index of the IP interface (only under an IES or VPRN service)

ascii-tuple

an ASCII-encoded concatenated tuple, consisting of [system-name | service-id | interface-name] (for VPRN or IES) or [system-name | service-id | sap-id] (for VPLS)

vlan-ascii-tuple

an ASCII-encoded concatenated tuple, consisting of the ascii-tuple followed by dot1p bits and dot1q tags

For VPRN, the ifindex is unique only within a VRF. The DHCP relay function automatically prepends the VRF ID to the ifindex before relaying a DHCP Request. VPRN is supported on the 7750 SR only.

When a DHCP packet is received with Option 82 information already present, the system can do one of three things. The available actions are:

Replace

On ingress, the existing information-option is replaced with the information-option parameter configured. On egress (toward the customer), the information option is stripped (per the RFC).

Drop

The DHCP packet is dropped and a counter is incremented.

Keep

The existing information is kept on the packet and the router does not add more information. On egress the information option is not stripped and is sent on to the downstream node.

In accordance with the RFC, the default behavior is to keep the existing information; except if the GIADDR of the packet received is identical to a local IP address on the router, then the packet is dropped and an error incremented regardless of the configured action.

The maximum packet size for a DHCP relay packet is 1500 bytes. If adding the Option 82 information would cause the packet to exceed this size, the DHCP client packet is discarded. This packet size limitation exists to ensure that there is no fragmentation on the end Ethernet segment where the DHCP server attaches.

In the downstream direction, the inserted Option 82 information should not be passed back toward the client (as per RFC 3046, DHCP Relay Agent Information Option). To enable downstream stripping of the Option 82 field, DHCP snooping should be enabled on the SDP or SAP connected to the DHCP server.